Side effects are not common with insulin, but they can occur. The most common side effects are related to low blood glucose (hypoglycemia) or high blood glucose (hyperglycemia).

Low blood glucose, or low blood sugar, can be caused by the interaction of the following factors:

• Food
• Exercise
• Medication

Too much or too little of any of these three factors can affect glucose levels. Low blood glucose can be triggered by:

• Not enough food.
• Exercising more than usual without taking an adequate meal/snack to cover that exercise.
• Drinking significant amounts of alcohol.
• Taking certain medicines.
• Using too much insulin or the wrong type.
• Sickness, particularly vomiting or diarrhea.

Symptoms differ from person to person but include:

• Shakiness
• Dizziness
• Rapid heartbeat
• Sweating
• Confusion
• Blurred vision
• Headache
• Numbness or tingling of the mouth
• Fatigue or weakness
• Paleness
• Hunger (polyphagia)

Diabetic individuals should have a physician-directed plan for treating hypoglycemia. In general, people who experience symptoms of hypoglycemia should immediately consume a food or beverage with quick-acting sugar and call their physician. Foods with quick-acting sugars include:

• Hard candy
• Fruit juice or nondiet soft drink (4 to 6 ounces)
• Glucose tablets or gel
• Corn syrup or honey (1 tablespoon)
• Sugar cubes (six half-inch cubes) or table sugar dissolved in water

Chocolate contains fat that slows sugar from entering the bloodstream.

High blood glucose is a serious problem that can lead to a bloodstream disorder called diabetic ketoacidosis. This involves a potentially life-threatening buildup of acids called ketones and can trigger a diabetic coma. The following conditions can trigger hyperglycemia:

• Diarrhea, fever or infection
• Lack of insulin or forgetting to take insulin medication
• Failure to exercise
• Overeating or not complying with the restrictions of a meal plan

Symptoms associated with high blood glucose tend to appear more slowly than those of low blood glucose. Diabetic patients should seek immediate medical care if they experience the following symptoms:

• Excessive thirst (polydipsia)
• Dry mouth
• Tiredness
• Fruit-like breath odor
• Flushing
• Dry skin
• Frequent urination (polyuria) and increased volume
• Unexplained loss of appetite, nausea or vomiting
• Breathing difficulties

A physician also should be informed if the following symptoms appear and are severe or persistent:

• Redness, swelling and itching at the injection site
• Changes in the skin, including swelling, depression at injection site or thickening
• Fat buildup or fat breakdown at the injection site

Immediate medical consultation is also necessary with the following symptoms:

• Seizures
• Loss of consciousness
• Severe sunburn
• Yellowing of the skin or eyes (jaundice)
• Light-colored stools
• Dark urine
• Unusual bleeding or bruising
• Fever
• Sore throat
• High levels of ketones in the urine (ketosis, a possible sign of ketoacidosis)

Some patients who use insulin may experience weight gain. This can cause additional health problems, particularly in patients with type 2 diabetes.  However, one recent two-year study of low-dose insulin prescribed to children at risk of type 1 diabetes found that the treatment caused no change in weight.

Other potential, but rare, side effects include loss of fatty tissue (lipodystrophy) at the site where the insulin is injected, and allergic reactions such as swelling (edema).